1. Field of the Invention
This invention relates to circuit protection devices comprising PTC conductive polymers.
2. Introduction to the Invention
Conductive polymer and ceramic compositions exhibiting PTC behavior, and electrical devices comprising them, are well known. Reference may be made, for example, to U.S. Pat. Nos. 2,952,761, 2,978,665, 3,243,753, 3,351,882, 3,571,777, 3,757,086, 3,793,716, 3,823,217, 3,858,144, 3,861,029, 3,950,604, 4,017,715, 4,068,281, 4,072,848, 4,085,286, 4,117,312, 4,177,376, 4,177,446, 4,188,276, 4,237,441, 4,242,573, 4,246,468, 4,250,400, 4,252,692, 4,255,698, 4,271,350, 4,272,471, 4,304,987, 4,309,596, 4,309,597, 4,314,230, 4,314,231, 4,315,237, 4,317,027, 4,318,881, 4,327,351, 4,330,704, 4,334,351, 4,352,083, 4,388,607, 4,398,084, 4,413,301, 4,425,397, 4,426,339, 4,426,633, 4,427,877, 4,435,639, 4,429,216, 4,442,139, 4,450,496, 4,459,473, 4,459,632, 4,475,012, 4,481,498, 4,476,450, 4,502,929, 4,514,620, 4,515,449, 4,534,889, 4,542,365, 4,545,926, 4,549,161, 4,560,498, 4,562,313, 4,647,894, 4,647,896, 4,685,025 and 4,689,475, and commonly assigned U.S. Ser. No. 103,077 (Fang, et al.), now abandoned in favor of a continuation application, Ser. No. 293,542, filed Jan. 3, 1989, and 115,089 filed by Fang, et al. on Oct. 30, 1987, now abandoned. The disclosure of each of the patents and applications referred to above is incorporated herein by reference.
Particularly useful devices comprising PTC conductive polymers are circuit protection devices. Such devices have a relatively low resistance under the normal operating conditions of the circuit, but are "tripped", i.e., converted into a high resistance state, when a fault condition, e.g., excessive current or temperature, occurs. When the device is tripped by excessive current, the current passing through the PTC element causes it to self heat to an elevated temperature at which it is in a high resistance state. Such devices, and PTC conductive polymer compositions for use in them, are described for example in U.S. Pat. Nos. 4,237,411, 4,238,812; 4,255,698; 4,315,237; 4,317,027; 4,329,726; 4,352,083; 4,413,301; 4,450,496; 4,475,138; 4,481,498; 4,534,889; 4,562,313; 4,647,894; 4,647,896; and 4,685,025 and in copending commonly assigned U.S. application Ser. Nos. 141,989, 711,909, now U.S. Pat. No. 4,774,024, Ser. No. 711,910, now U.S. Pat. No. 4,724,417, and Ser. No. 103,077, now abandoned. When the circuit protection device is "tripped", a thermal gradient is created. Where the thermal gradient flows in the same direction as the current flow, measures can be taken to assure that the peak temperature of the thermal gradient, i.e. the "hotline" or "hotzone" does not form near an electrode. Such preventative measures are described in U.S. Pat. Nos. 4,317,027 and 4,352,083. The disclosure of each of these patents and pending applications is incorporated herein by reference.
A particularly important use for circuit protection devices is in telecommunications apparatus, which can be exposed to a variety of different fault conditions. Reference may be made for example to U.S. Pat. Nos. 4,068,277, 4,068,281, 4,475,012, 4,459,632, 4,562,313, 4,647,894, 4,647,896 and 4,685,025, and application Ser. No. 711,909, now U.S. Pat. No. 4,774,024, Ser. No. 711,910, now U.S. Pat. No. 4,724,417, and Ser. No. 103,077, now abandoned, the disclosures of which are incorporated herein by reference.